FIELD OF THE INVENTION
The present invention relates to a horizontal oscillation control circuit for a cathode-ray tube of a direct driving system.
FIG. 1 is a block diagram showing a conventional horizontal oscillation control circuit shown in a Japanese patent, laid open No. '89-101776, for example. In the figure, 1 is a first monostable multivibrator (hereinafter referred to as an M/M) which outputs a pulse signal synchronized with a horizontal synchronizing signal, and 2 is a second M/M which outputs horizontal drive pulses being synchronized with the pulse signal output from the first M/M to be supplied to the next horizontal drive circuit.
A part 3 is a shaping circuit for shaping the waveform of a flyback pulse (hereinafter referred to as a FBP), and 4 is a phase comparator which compares the phase of a waveform-shaped FBP and that of an input horizontal synchronizing signal, and outputs a digital output corresponding to the phase difference. A part 5 is a charge pump which converts the output of the phase comparator 4 to an analog output, and 6 is a low pass filter (hereinafter referred to as an LPF) which removes the AC component and the noise in the output of the charge pump 5 and supplies the output signal to the first M/M 1.
A part 7 is an integration circuit for integrating a horizontal drive pulse signal output from the second M/M 2, and 8 is a differential amplifier which compares the output of the integration circuit 7 and a reference level and supplies the differential output to the second M/M 2 for controlling the pulse width of the horizontal drive pulse signal.
Next, the operation will be explained. The FBP from a horizontal output circuit, whose drawing is omitted, is converted to a TTL level through waveform-shaping by a shaping circuit 3. The waveform-shaped FBP is input to the phase comparator 4 together with a horizontal synchronizing signal which is input from the exterior. The phase comparator 4 compares these signals and outputs a digital signal having a pulse width corresponding to the phase difference. When the input horizontal synchronizing signal leads the FBP, the phase comparator outputs a S1 signal to the charge pump 5, and when the input horizontal synchronizing signal lags behind the FBP, it outputs a S2 signal to the charge pump 5. When the S1 signal is input to the charge pump 5, it is controlled to raise the output and when the S2 signal is input to it, it is controlled to lower the output. The output of the charge pump 5 is input to the LPF 6 and the AC component and noise are eliminated to be a DC voltage and it is input to the first M/M 1 as a bias voltage which specifies the output pulse-width of the first M/M 1. Therefore, when the horizontal synchronizing signal leads the FBP, the pulse width of a pulse signal output from the first M/M 1 is narrowed and the FBP is adjusted to be in phase with the horizontal synchronizing signal. In contrast with this, when the horizontal synchronizing signal lags behind the FBP the pulse width of the pulse signal output from the first M/M 1 is widened and the FBP is adjusted to be in phase with the horizontal synchronizing signal. The trailing edges of the horizontal synchronizing signal and those of the FBP are adjusted to coincide with each other as shown in FIG. 2, for example.
On the other hand, when the frequency of the horizontal synchronizing signal becomes higher, the differential output voltage of the differential amplifier 8, in which the output of the integration circuit 7 integrating the horizontal drive pulse signal is compared with the reference level, becomes higher, so that the pulse width of the horizontal drive pulse signal output from the second M/M 2 is narrowed.
As described in the above, the phase and the pulse width of the horizontal drive pulse are controlled corresponding to the frequency of the input horizontal synchronizing signal; thereby, even when the frequency of the horizontal synchronizing signal is switched, normal driving is possible and the picture can be displayed in a fixed range.
Since a conventional horizontal oscillation control circuit is constituted as mentioned in the above, a horizontal drive pulse signal in phase with the input horizontal synchronizing signal is output at all times. In other words, as shown in FIG. 2, the horizontal synchronizing signal and the horizontal drive pulse signal (output of the second M/M 2) are overlapped with one another. Therefore, the picture position cannot be freely adjusted, and it is difficult to absorb the difference in timing caused by the difference in manufacturers or types of equipment. The control is executed by an analog control, which increases the number of parts and requires a large mounting space; moreover, there has been a problem that the control cannot correspond to a microcomputer control.